1. Field of the Invention
This invention relates to probes and sensing devices for sensing EMF changes in a process environment, and more particularly, to probes and sensors of the type which contain a reference electrode and a second electrode spaced therefrom, and which detect or determine changes in a property of a process medium adjacent to at least one of the electrodes.
2. Brief Description of the Prior Art
A number of types of probes and sensor devices have been proposed for electronically monitoring or sensing changes in process conditions and parameters. These have been used for various purposes, including, for example, measuring changes in the oxygen content of a solution, determining hydrogen content of high-temperature fluids and monitoring corrosion in a system. One general type of probe used for these purposes includes a reference electrode and a second electrode, sometimes called a working electrode, both positioned in spaced relation in a housing of some type, and connected to a potential-measuring device. Change in the EMF across the electrodes is indicative of a change in a particular process condition monitored.
A number of criteria have been used to evaluate the effectiveness of such probes or sensor devices, including the speed of response (how quickly the process change is sensed and indicated), the cost or expense of constructing the probe or sensor, its mechanical durability, and its universality (ability to be used, for example, in both the gas, liquid, or mixed phase systems). Some types of probes previously used have been relatively narrow in their applicability and usefulness, and others have had a slow or delayed response time which is acceptable in some usage, but unsatisfactory in others. One type of electronic hydrogen sensor now in use for corrosion monitoring works well in selectively detecting process changes which cause corrosion. When a film inhibiting the sensitivity of the probe forms on the steel hydrogen diffusion tube of the sensor, the detection is delayed in its response to process changes which result in corrosion. This represents corrosion to be experienced by the process vessel, but does not serve as an early warning of possible corrosion.
Many of the types of electrodes previously in use are expensively constructed, employing platinum tubes and wires, and/or expensive chemical packings, or costly catalytic coatings on noble metals.
Examples of some of the dual electrode type probes or sensing devices recently proposed and patented include those shown in U.S. Pat. No. 3,657,096 to Proctor; U.S. Pat. No. 3,510,420 to Mills; U.S. Pat. No. 3,647,641 to Grubb et al.; U.S. Pat. No. 3,661,749 to Richardson; U.S. Pat. No. 3,649,473 to Holden et al.; U.S. Pat. No. 3,328,277 to Solomons et al., and U.S. Pat. No. 2,913,386 to Clark.